即使方法使用更多泛型类型，方法调用也不匹配方法签名 [英] Method call doesn't match method signature even though method is using more generic types

问题描述

方法:

public static void incrementMapCounter( Map<Object,Number> tabulationMap, Object key ) {
  Number value = 0;
  if ( tabulationMap.containsKey(key) ) {
    value = tabulationMap.get(key);
  }
  value = value.doubleValue() + new Double(1);
  tabulationMap.put( key, value );
}

调用该方法:

Map<String,Long> counts = new HashMap<>();
String key = "foo-bar";
incrementMapCounter( counts, key );

错误(重新格式化):

The method
    incrementMapCounter(Map<Object,Number>, Object)
in ... is not applicable
    for the arguments  (Map<String,Long>, String)

方法签名是匹配类型或更通用的:

The method signature is either a matching type or more generic:

• 地图是地图
• 字符串是一个对象(x2)
• 长是数字

我对此有些困惑.

推荐答案

是后面的两个.字符串和对象不是同一类型.泛型不是协变的，它们是不变的.类型必须完全匹配.与Long和Number相同.

It's the later two. String and Object are not the same type. Generics are not covariant, they are invariant. The types have to match exactly. Same with Long and Number.

对于方法签名，您可以尝试:

For your method signature you might try:

public static <T> void incrementMapCounter( Map<? extends T, ? extends Number> map, T key )
{ ...

可以通过以下方式致电

 HashMap<String, Integer> myMap = new HashMap<>();
 incrementMapCounter( myMap, "warble" );

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